The present invention relates to a method for suppressing the influence of roll eccentricities on a feedback control for a rolling-stock thickness in a roll stand, an insensitive or non-responsive dead zone being provided in the thickness control for signal fluctuations caused by the roll eccentricities, and the zone width of this insensitive dead zone being varied relative to the magnitude of the signal fluctuations.
In controlling the thickness of the rolling stock in a roll stand, one encounters the difficulty that the thickness of the rolling stock cannot be easily measured as a controlled variable of interest that is able to be evaluated using control engineering methods at the location where it originates, namely the roll nip and, therefore, cannot be drawn upon to immediately correct disturbances, such as roll eccentricities. However, in accordance with the so-called gauge-meter principle, the thickness h.sub.a of the rolling stock as it emerges from the roll nip can be determined arithmetically from the setting position s of the rolls, the roll separating force F.sub.W and the spring constants c.sub.G of the roll stand as EQU h.sub.a +.DELTA.R=s+F.sub.W /c.sub.G
In the case of the so-called AGC (automatic gauge control) method, proceeding from this relation the roll separating force is detected by means of a roll-load detector and is drawn upon to control the thickness of the rolling stock. If the roll nip becomes larger, for example because of an increase in the feed thickness of the rolling stock, this leads to an increase in the roll separating force; this increase is detected, the setting position s of the rolls being reduced by means of the control, so that the roll separating force F.sub.W is increased further and the thickness of the rolling stock is again readjusted to its setpoint value. However, as equation (1) shows, the rolling-stock thickness h.sub.a is not available by itself, but rather only together with the roll eccentricity .DELTA.R, which causes a periodic increase and decrease in the roll separating force F.sub.W during the rolling process. However, the increase and decrease in the roll separating force F.sub.W caused by the eccentricities are mistakenly interpreted by the AGC system as an increase or decrease in the roll nip, through which means the roll separating force F.sub.W is automatically increased or reduced by way of the setting position s, and the eccentricities are consequently rolled in their entirety into the rolling stock. To prevent this negative effect that the roll eccentricities have on the feedback control of the rolling-stock thickness, the German Patent 26 43 686 discloses providing a dead zone in the control that is insensitive to the signal fluctuations produced by the roll eccentricities. In this case, the width of the dead zone is varied in dependence upon the amplitudes of the signal fluctuations and is thus adapted to the extent of the eccentricities.
The German Patent Application P 42 31 615.4 proposes varying the zone width in dependence upon an ongoing statistical evaluation of the signal fluctuations, their standard deviation preferably being determined.
In these proposed solutions, since all the signal fluctuations are interpreted as instances of eccentricity, actual fluctuations in the thickness of the rolling stock lead to an increase in the dead-zone width and, thus, to a slower correction of the thickness error.
The invention improves upon the prior art by optimizing the adaptation of the dead-zone width in dependence upon the occurring eccentricies.